An electronically commutated motor (ECM), also referred to as a brush less DC motor, has permanent magnets mounted on its rotor. The stator of an ECM has teeth and coil windings wound thereon, which interact with the permanent magnet rotor to produce positive or negative torque, depending on the direction of current therein with respect to the magnetic polarities. An electronic inverter is employed to energize the stator windings and thus control the amount and direction of torque as well as the rotor shaft speed.
Single-phase ECM's typically have starting problems. The magnetic saliencies caused by the stator teeth result in a cogging, or indenting, torque which forces the permanent magnet rotor to rest, or park, at particular positions. Unfortunately, in single-phase ECM's, the parking positions may coincide with positions of zero electromechanical torque production, thus making it difficult to start. One way to overcome this problem is to provide a starting winding. Another approach is to employ parking cuts in the stator. Parking cuts provide cogging torques adjusted for stopping the rotor at predetermined positions wherein there is sufficient torque to start the motor. Disadvantageously, however, such parking cuts typically reduce electromagnetic torque production and efficiency as well as introduce torque pulsations and noise.
Accordingly, it is desirable to provide an ECM having improved starting capability, while producing high torque output at high efficiency and low noise.